We had this same problem, it was very intermittent at first but as gradually got worse until a few days ago became unwatchable. We could still watch BBC1 via terrestrial Tv so we knew the Tv was ok.
The sky engineer has just left and all is well now.
Hopefully the following is useful to anyone else having the same issues.
FYI. Live TV is currently transmitted via your dish and cables. Catch up and streaming services come via your broadband.
In our case the engineer removed the cables from the back of the box and he could immediately see that one of the copper ends was corroded and one of the box connectors was corroded too.
So water/ moisture has got into the cable and through gravity eventually found it's way into the house and sky box.
So the cables need replacing. If they do this for you ensure the new cables at the dish end are secured to the dish bracket and not just left hanging free as they were with ours. Also don't let them just cut off an inch or two from the cable ends and put new connectors on, or the problem will sooner or later return. Make sure they replace the whole cables, and given there could be internal corrosion I'd push for a new box too even if it looks ok, after all moisture has got into your box through one or both of those connectors.
Hope this helps.
